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xen-bus/block: explicitly assign event channels to an AioContext

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It is not safe to close an event channel from the QEMU main thread when
that channel's poller is running in IOThread context.

This patch adds a new xen_device_set_event_channel_context() function
to explicitly assign the channel AioContext, and modifies
xen_device_bind_event_channel() to initially assign the channel's poller
to the QEMU main thread context. The code in xen-block's dataplane is
then modified to assign the channel to IOThread context during
xen_block_dataplane_start() and de-assign it during in
xen_block_dataplane_stop(), such that the channel is always assigned
back to main thread context before it is closed. aio_set_fd_handler()
already deals with all the necessary synchronization when moving an fd
between AioContext-s so no extra code is needed to manage this.

Reported-by: Julien Grall <jgrall@amazon.com>
Signed-off-by: Paul Durrant <pdurrant@amazon.com>
Reviewed-by: Anthony PERARD <anthony.perard@citrix.com>
Message-Id: <20191216143451.19024-1-pdurrant@amazon.com>
Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
This commit is contained in:
Paul Durrant 2019-12-16 14:34:51 +00:00 committed by Anthony PERARD
parent b8030af450
commit 32d0b7be68
3 changed files with 45 additions and 7 deletions
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20
hw/block/dataplane/xen-block.c
View File

@ -685,12 +685,24 @@ void xen_block_dataplane_stop(XenBlockDataPlane *dataplane)
return;
}
xendev = dataplane->xendev;
aio_context_acquire(dataplane->ctx);
if (dataplane->event_channel) {
/* Only reason for failure is a NULL channel */
xen_device_set_event_channel_context(xendev, dataplane->event_channel,
qemu_get_aio_context(),
&error_abort);
}
/* Xen doesn't have multiple users for nodes, so this can't fail */
blk_set_aio_context(dataplane->blk, qemu_get_aio_context(), &error_abort);
aio_context_release(dataplane->ctx);
xendev = dataplane->xendev;
/*
* Now that the context has been moved onto the main thread, cancel
* further processing.
*/
qemu_bh_cancel(dataplane->bh);
if (dataplane->event_channel) {
Error *local_err = NULL;
@ -807,7 +819,7 @@ void xen_block_dataplane_start(XenBlockDataPlane *dataplane,
}
dataplane->event_channel =
xen_device_bind_event_channel(xendev, dataplane->ctx, event_channel,
xen_device_bind_event_channel(xendev, event_channel,
xen_block_dataplane_event, dataplane,
&local_err);
if (local_err) {
@ -818,7 +830,11 @@ void xen_block_dataplane_start(XenBlockDataPlane *dataplane,
aio_context_acquire(dataplane->ctx);
/* If other users keep the BlockBackend in the iothread, that's ok */
blk_set_aio_context(dataplane->blk, dataplane->ctx, NULL);
/* Only reason for failure is a NULL channel */
xen_device_set_event_channel_context(xendev, dataplane->event_channel,
dataplane->ctx, &error_abort);
aio_context_release(dataplane->ctx);
return;
stop:

27
hw/xen/xen-bus.c
View File

@ -1089,8 +1089,26 @@ static void xen_device_event(void *opaque)
}
}
void xen_device_set_event_channel_context(XenDevice *xendev,
XenEventChannel *channel,
AioContext *ctx,
Error **errp)
{
if (!channel) {
error_setg(errp, "bad channel");
return;
}
if (channel->ctx)
aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
NULL, NULL, NULL, NULL);
channel->ctx = ctx;
aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
xen_device_event, NULL, xen_device_poll, channel);
}
XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
AioContext *ctx,
unsigned int port,
XenEventHandler handler,
void *opaque, Error **errp)
@ -1116,9 +1134,10 @@ XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
channel->handler = handler;
channel->opaque = opaque;
channel->ctx = ctx;
aio_set_fd_handler(channel->ctx, xenevtchn_fd(channel->xeh), true,
xen_device_event, NULL, xen_device_poll, channel);
/* Only reason for failure is a NULL channel */
xen_device_set_event_channel_context(xendev, channel,
qemu_get_aio_context(),
&error_abort);
QLIST_INSERT_HEAD(&xendev->event_channels, channel, list);

5
include/hw/xen/xen-bus.h
View File

@ -128,10 +128,13 @@ void xen_device_copy_grant_refs(XenDevice *xendev, bool to_domain,
typedef bool (*XenEventHandler)(void *opaque);
XenEventChannel *xen_device_bind_event_channel(XenDevice *xendev,
AioContext *ctx,
unsigned int port,
XenEventHandler handler,
void *opaque, Error **errp);
void xen_device_set_event_channel_context(XenDevice *xendev,
XenEventChannel *channel,
AioContext *ctx,
Error **errp);
void xen_device_notify_event_channel(XenDevice *xendev,
XenEventChannel *channel,
Error **errp);